The process of aging has always intrigued people. The conundrum related to the reasons for aging, the timing for aging, the aging marker, and whether or not there is a limit for people to grow old has created a mystery that has been dwelt upon by the mankind for hundreds of years. Despite the advancement made in the study of genetics and molecular biology, the mysteries surrounding the aging body are yet to be unfolded. There are several theories of aging, which try to explain why the body organs, skin and sensory capabilities degrade with age. However, all theories have some limitations and none of those theories are able to come up with a comprehensive solution which provides explains all aging process changes. This essay will discuss and compare different theories of aging and explore the changes experienced by an aging body and the possible reasons for that.
Theories of Biological Aging
There are many theories available for biological aging. According to the traditional aging theory, the process of aging is not genetically programmed. However, modern biological aging theories divide the process of human aging into two categories, programmed and damage or error theories (Jin, 2010). As per the programmed theories, aging pursues a biological timetable, which is possibly a continuation of the one that regulates the growth and development during childhood. This regulation is based on the changes that take place in gene expression affecting the systems accountable for the maintenance, repair and defense responses. The theories of damage or error, on the other hand, cite environmental assaults that bring about cumulative damages to the living organism as the cause of aging (Jin, 2010).
There are three sub-categories to the programmed theory of aging:
1) Programmed Longevity: As per programmed longevity, aging is an outcome of a sequential turning on and off of a set of genes, with age-related deterioration being defined as the time when age-related deficits surface (Davidovic et al, 2010).
2) Endocrine Theory: According to this theory, hormones regulate the biological clock, controlling the pace of aging. Recent studies also confirm the same that the process of aging is regulated by hormones; especially the insulin/IGF-1 signaling (IIS) pathway is crucial to the hormonal regulation of aging (Heemst, 2010). For instance, estrogen is a female hormone that is produced in abundance in the ovaries of women, and it is the main hormone that helps women remain youthful. However, it is believed that after menopause, the estrogen production reduces or completely stops in women, leading to wrinkle formation and aging.
3) Immunological Theory: This theory implies that the system of immunity is programmed to decline after a certain time, leading to an increased risk of infectious diseases, aging and death. The fact that the effectiveness of the immune system reaches its peak at puberty and then declines gradually over time with aging is well-documented. For example, as one begins to age, the antibodies lose their potency, with the body being capable of fighting fewer new diseases, leading to eventual death (Cornelius, 1972).
The damage or error theory is divided into five sub-categories:
1) Wear and Tear Theory: According to this theory, just as a machine wears out with time from repeated use, human body too wears out over time (Brys, Vanfleteren and Braeckman, 2007). For example, the eyes get worn-out due to repeated use and lose their function over time, and the same thing happens to every organ in the body.
2) Rate of Living Theory: This theory explains that the shorter the lifespan of an animal, the greater its rate of oxygen basal metabolism. It has been observed that animals with higher BMR have lower life span than animals with lower BMR per unit weight. For instance, dogs that have a shorter life spans than human beings have higher BMR (53 kcal/kg/day) than that of human beings (30 kcal/kg/day). However, this theory cannot adequately explain the maximum life span of a living organism (Brys, Vanfleteren and Braeckman, 2007).
3) Cross-linking Theory: According to this theory, the cells and tissues of a living organism are damaged by the build-up of cross-linked proteins, slowing down the system leading to aging (Bjorksten and Tenhu, 1990).
4) Free Radicals Theory: This theory explains that superoxide and other free radicals damage the macromolecular components of the cell, such as sugars, nucleic acids, proteins, and lipids, leading to accumulated damage-causing cells, and gradually, various organs of the body stop functioning (Gerschman, Gilbert, Nye and Dwyer, 1954).
5) Somatic DNA Damage Theory: As per this theory, DNA damages are continuous in the cells of a living organism. Though most of these damages are repaired, some damages accumulate over time as the repair mechanisms are unable to rectify the defects as fast as these are produced. Therefore, genetic mutations and accumulations occur, leading to deterioration and malfunctioning of the cells (Jin, 2010).
The sensory capacity refers to the ability to interact with the environment through the five senses, including vision, smell, hearing, taste, and peripheral sensation. However, with aging, this ability to perceive the world through the senses deteriorates due to changes in the sensory functions. For instance, changes in hearing can affect the communication skill of an elderly person, or changes in vision can make simple thing like taking medications a difficult problem. Changes in the sensory capacity are a normal part of aging, with the majority of older adults experiencing the deterioration of their senses. 30% adults over the age of 65 experience some level of visual impairment, with cataracts being the 5th most common condition in adults aged over 75 (Cacchione, 2014). The condition of Presbyopia in which the elasticity in the eye lenses is lost, leading to a decreased ability to change the shape of the lens to focus on near objects is common among older adults.
Some of the vision problems that afflict an aging person include cataracts; macular degeneration, the most common cause of legal blindness in the older people; glaucoma, a serious form of eye disease that leads to the loss of peripheral vision, decreased ability to adapt to the dark, and intolerance of glare; diabetic retinopathy, which, triggered by diabetes, causes spotty vision; hypertensive retinopathy; temporal arteritis, which causes inflammation of the temporal artery and sudden vision loss; and detached retina that can affect cataract patients (Salvi, Akhtar, and Currie, 2006).
Hearing loss, which is the 3rd chronic condition the aging adults suffer from, afflicts the majority of people aged over 75 years. The most common changes in hearing experienced by the elderly include Presbycusis, which brings changes in the blood supply of the cochlea, making it difficult to hear high pitched sound; conductive hearing loss, which involves damage to the outer or middle ear; sensorineural hearing loss, which involves the damage to the cochlea and the fiber of the 8th cranial nerve (Cacchione, 2014).
Though changes in smell, which lead to the decreased ability to identify odors, are common among older adults, it is not, however, regarded as a normal part of aging. Changes in the smell are especially related to neurological conditions like Alzheimer's disease and Parkinson's disease (Davidovic et al, 2010).
Common changes in taste include a reduced number of taste buds and less saliva production. However, though changes in taste are common, they are more often the outcome of dental problems and medications. In fact, research says that if proper care is taken, then the number of taste buds on our tongue will not decrease (Schwartz, 2005). The only reason for the worse functioning of taste buds and the decrease in the number of taste buds can be attributed to environmental factors like low maintenance, over consumption of medication, and medical problems. There is no intrinsic factor that causes the degradation of our tasting capability with age (Schwartz, 2005).
The function of peripheral nerve that regulates the sense of touch decreases with age. Though the ability to understand painful stimuli is retained in old age, the reaction time to painful stimuli may become slower.
The common sense tells that this reduced functional capability of the sensory organs happens because of the aging process. The overuse of the organs leads to degradation over time. Most of theories support that too. However, a few scientists like David Zacks feel that most of the sensory organs can remain healthy and perfect with proper care taken in old age (Salvi, Akhtar, and Currie, 2006). For example, in the case of the eye, the main degradation happens only in the corneal layer, because of the environmental factor as the thin layer gets scratched in the same fashion as does a camera lens when used for a prolonged period of time (Salvi, Akhtar, and Currie, 2006). However, he believes that there is no intrinsic reason for aging for the sensory organs as predicted by some of the theories.
Another major problem in older age as we have seen above is the loss of balance, because of problems in the ear muscles. The reason for the loss of balance in old age is caused by the loss of muscle coordination or deterioration of ear organ. However, there are evidences found in recent times, which, if practiced properly, actually can improve the balance even in old age instead of degradation. This is a conflicting finding from most of the theoretical conclusions about old age sensory capability disability.
Brain and Neural Systems
Brain and neural system grows between the ages 0 and 20. However, after the age of 20, the growth of neurons in the brain stops. There is empirical evidence that the brain starts shrinking after the age of 65 (Berchicci, Lucci and Di Russo, 2013). There is strong evidence that the number of neurons in our brain starts decreasing from the late 20s itself. The number of white cells also starts decreasing after the age of 50. Till our 20s, we are able to learn new things faster and also our brain is able to process information faster. However, from the 30s onwards, some people may begin to forget small things as the number of neurons in our brains starts to decrease (Berchicci, Lucci and Di Russo, 2013). The process further deteriorates as we age. However, the reason for that is not really known.
Radical theory is able to somewhat explain the aging process of the brain using the oxidation process. According to the radical theory, as a cell goes through oxidation process, it leaves unnecessary chemicals and materials in the cells, leading to the degradation of the brain cells, and in fact, the generation of new cells completely stops after a certain age. But this theory is unable to explain why the aging process suddenly starts during the 20s (Berchicci, Lucci and Di Russo, 2013). Similarly, other theories also are able to explain only a part of the degradation of the neural system. Skin and organ degradation are more prevalent and noticeable than the brain degradation. In fact, on many occasions, it has been noticed that for some people, the brain starts to shrink much earlier than others. This is often attributed to different genes. The critics of the aging process say that if different genes can determine when the neural system is going to age, then it is possible that with advancements in the gene technology, it will be easier to understand which structure is responsible for the aging of the brain so that necessary course of action can be taken to stop the aging of brain and neural system (Berchicci, Lucci and DI Russo, 2013).
Though there are many theories available as regards the process of aging, none of them gives a satisfactory explanation. Each of the theory is different from one another, and each theory cites different reasons for the aging process. Hence, despite multiple theories proposed by many scholars, due to the lack of consensus on any one theory, no theory can be regarded as concrete and full-proof. Also, different parts of our body age differently. For example, skin shows the effects of aging more consistently among all the humans than other parts of the body. Eye, for example, shows varying levels of functioning at old age. Some people retain excellent eyesight whereas some other people develop cataract and other degenerative eye problems. Brain is also another part of the body that degrades as we age consistently. For some, it may start at a younger age than others, but it ages with every passing year. Although there are some critics of the aging process, but by and large, it is established that aging happens to all our body parts. However, no theory is able to explain the reason consistently. Further researches need to be done upon all the theories to arrive at a satisfactory explanation of aging.
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